A potentiometric electrically heated exhaust-gas probe is disclosed, for example, in U.S. Pat. No. 4,310,401. This probe is used to control combustion processes such as in heater equipment or in internal combustion engines for motor vehicles. For motor vehicles, various countries have statutory requirements as to on-board monitoring of all exhaust-gas relevant components. A fault which can lead to a deterioration of the exhaust gas must be detected and displayed via a fault lamp. In this context, the invention relates especially to the diagnosis of all electrical faults of the exhaust-gas probe during its operation. As electrical fault, especially short circuits of the connecting leads of the probe are noted, such as a short circuit to the battery voltage, a short circuit to ground, a break in the cable, et cetera.
The control of combustion processes takes place often with a so-called two-point control strategy. Here, the probe is subjected to the exhaust gas of the combustion process and distinguishes between oxygen rich and oxygen deficient exhaust gas. For oxygen-rich exhaust gas, the air/fuel mixture, which is supplied to the combustion process, is enriched with fuel until the resulting exhaust gas is oxygen deficient. Thereafter, a leaning of the mixture takes place by reducing the metering of fuel until the occurrence of an oxygen excess in the exhaust gas. In this way, the composition of the combusting air/fuel mixture varies periodically between oxygen deficiency and oxygen excess. The probe signal changes in fault-free probe operation back and forth between a first region of high signal values (oxygen deficiency) and a second region of low signal values (oxygen excess). These two regions are separated by a third region which is run through very rapidly for each change between the first and second regions when the probe operation is free of faults.
A known probe diagnosis evaluates a longer dwell time of the probe signal in the above-mentioned third region as a fault because this performance is typical for an electrical fault such as a break in the signal and ground leads between the probe and the control apparatus. This diagnosis has been shown to be especially reliable for specific types of probes but has shown deficiencies with respect to other types of probes. Especially in the case of planar lambda=1 probes such as disclosed in U.S. Pat. No. 4,310,401, cable breaks in experimental operation have not been detected with sufficient reliability.